Various devices are used to obtain hard copy readout of graphic information produced by computers. If the graphics (pictures) are in color, a popular device for obtaining the hard copy is a colorgraphics film recorder, which in its simplest form is merely an ordinary camera used to photograph the cathode ray tube (CRT) screen of a computer monitor. This is often inadequate because monitor CRTs have limited resolution and usually have a limited ability to show a wide range of colors.
Colorgraphics film recorders of more sophisticated design utilize a high resolution CRT within a recorder cabinet and a camera which photographs the display on the CRT. Since the generation of the graphics display on the screen may be done more leisurely than when displays are to be viewed directly, the color fidelity and the range of colors produced by these internal CRTs can be greater than that provided by monitor CRTs. The improved resolution is achieved by utilizing monochrome CRTs in combination with color filters. Monochrome CRTs are less expensive and have substantially higher resolution. The graphics computer provides numerical information to the recorder indicating the amounts of red, green, and blue to be displayed for each of a million or more dots or pixels comprising the picture. The recorder displays the red, green, and blue components of the complete pictures on the CRT screen in sequence by interposing red, green, and blue filters between the CRT and the color film on which the picture is being recorded. In this manner, full color photographs are obtained from a CRT that provides white light.
The manner of displaying the picture on the screen employed by colorgraphics film recorders is relatively slow and, because of the way the image is formed, does not have the maximum possible resolution. The picture display is produced as a sequence of vertical columns of pixels or of horizontal rows of pixels. Typically, one column of pixels is energized on the screen, a pixel at a time, with the brightness of each controlled according to a sequence of data bits provided by the external graphics computer. The column of pixels at the left-hand side of the screen is illuminated first. This may be done while, for example, a blue filter is interposed in front of the camera lens. A first sequence of data bits represents the amount of blue color to be imaged in the first column of the photograph. Then the same column position on the screen is again addressed, pixel by pixel, while a green filter is interposed in front of the lens, with the brightness of each pixel controlled according to a second series of data bits representing the amount of green light to be imaged on each individual pixel. The same column on the screen is then again addressed while a red filter is interposed in front of the lens, this time with the brightness of each pixel in the column controlled in accordance with a third series of data bits received from the computer, representing the amount of red light to be imaged for each pixel in the columns. Following this, the CRT beam is horizontally deflected one column, and the process is repeated, with the intensity or duration of illumination of each pixel dependent on the magnitude of a continuing stream of bits representing the intensity of each color needed for each pixel in the second column. The process continues in this way until the complete picture has been illuminated in three colors and the photographic film is completely exposed. In these conventional graphic recorders, the horizontal position of each column of pixels on the film is determined by the horizontal deflection of the CRT beam, which is indexed horizontally one pixel width following each exposure of a picture column in the three colors.
Typically a color wheel composed of three pie slice shaped filters is used which is rotated at a constant speed. The speed is slow enough that in the worst case (when all pixels in a column need full illumination) the exposure can be completed before the filter color changes. No overall time savings can be realized by speeding up the exposure of individual pixels because the filter is spinning at a constant speed. Picture regions needing little or no exposure will take the same length of time to illuminate as regions needing full exposure because it is necessary to wait until the next filter color is in place before proceeding with the exposure of the next color.
Resolution is limited in these conventional systems because the column on the CRT face having the least number of pixels limits the number of pixels per column, since each column must have the same number of pixels. Thus, the number of pixels in each column is equal to the number of pixels in the columns near the edge of the CRT. This number is substantially smaller than the number of pixels in a column near the center of a round CRT screen, which is the type typically used. On a picture having a length to width aspect ratio of 5:4, the number of pixels near the edge is 66% of the number of pixels in the center column. Thus the resolution of the image is reduced from what could be obtained if all of the pixels at the center of the screen were available.
Large non-linearities in the vertical displacement of pixels occurs when the CRT beam must undergo a large horizontal deflection. Thus, in the regions near the edge of the CRT, the vertical displacement must be corrected to prevent inaccuracies in pixel positioning. However, due to the non-linearities, it is often difficult to provide precise correction, particularly if the correction is based solely on a calculated (not measured) value.